Just to clarify: AMD recently updated its range, and changed its numbering system they made some changes to the cards but they are essentially rebrands and use the earlier GPU chips.
So, the R9 270X is, to all practical intents and purposes a HD7870GHz, the R9 280 is the HD7950 Boost and the R9 280X the faithful HD7970, the main difference is the inclusion of Trueaudio.
I'd say you're on the money with either the R9 270X or HD7950, for a new system I'd opt for the 7950/R9 280, the extra 1Gb of memory they carry is already handy in some games and is going to become essential for high settings in the future.
Don't look at the R9 280X or HD7970 unless they're cheap, a FX6300 is already on its limits with the HD7950, anything faster would just be wasted money (CPU bottleneck).
As always when purchasing off E-bay, check the sellers feedback, long established traders with plenty of positive feedback are generally trustworthy and are more likely to be helpful if a problem arises.